We have recently discovered that most microrganisms contain the enzymatic machinery for conjugation of cellular thiols with environmental toxicants (S-conjugation for short). Our immediate goal is to unravel the essential details of microbial S-conjugation so that an assessment of the environmental impact of the process can be made. Our specific aims are: 1) using a representative collection of aquatic and terrestrial microorganisms, including bacteria, fungi, algae, and protozoa, we will screen for the presence of S-conjugation enzymes using a range of electrophillic substrates and cellular thiols. 2) In suitable microbes we will determine the metabolic pathway(s) responsible for the production of S-conjugates; for example, we will determine the metabolic sequence of thiomethylation of the fungicide pentachloronitrobenzene to pentachlorothioanisole in Tetrahymena thermophila. 3) We will characterize the enzymes of S-conjugation pathway with special emphasis on determining their specificity for xenobiotics. 4) Promising strains will be incubated with selected environmental toxicants to search for the excretion and/or bioaccumulation of S-conjugates and related metabolites. 5) We will search for an S-conjugation system that will metabolize recalcitrant, lipophilic halogenated toxicants like lindane and hexachlorobenzene to water soluble sulfur-containing metabolites. 6) Finally, where new, novel sulfur-containing metabolites are found and identified preliminary short term toxicity and mutagenicity tests will be carried out to determine if microbial S-conjugation results in detoxification or toxification.